In metallurgical vessels having bottom electrodes, for instance plasma melt furnaces or vacuum arc furnaces, the electrical contact with the melt is produced by electrodes which are introduced in the refractory lining of the vessel. Such electrodes are, in this type of application, particularly subject to thermal wear. In order to reduce this thermal wear, the metal electrodes are typically cooled.
U.S. Pat. No. 4,435,812 shows the use of gases, and particularly air, as a cooling fluid. The disadvantages of this type of cooling, however, is the limited cooling efficiency occasioned by the use of air as the cooling medium.
In order to increase the removal of heat, liquids have also been considered as coolants. Thus, U.S. Pat. No. 4,423,512 discloses a bar-shaped electrode through which cooling water is introduced under pressure via a feed tube into a cavity in the electrode. The disadvantage of this prior art device, however, is that the cooling water is conducted up into the furnace vessel. In the event of possible negative situations occurring like, e.g., overheating of the electrode and melting of the closure part facing the melt, or the occurrence of cracks in the solid metallic part extending up to the water-cooled part of the electrode, the cooling water, which is under pressure, could penetrate below the molten bath and into the hearth. The result would be a gas explosion. Consequently, this device is potentially dangerous to the operating personnel and could cause destruction of the furnace.
Moreover, in the aforementioned '225 European patent, a layer of metal having low thermal conductivity and a low melting point is provided in an expensive fashion, by being positioned between the wear part and the cooling part of the bottom electrode. An electrode of this construction impedes the flow of heat and current and can, furthermore, not be used again, not even in parts.
Federal Republic of Germany 38 35 785 shows a bottom electrode for electric melting furnaces in which a cap is detachably connected to the wear part. The bottom electrode disclosed in this patent not only has low cooling capacity as a consequence of the cap, so that other cooling devices are necessary, but, in addition, means for the rapid replacement of the electrode.
In addition to the disadvantageously expensive work required for the replacement of the electrode shown in the '785 patent, this reference also fails to disclose mechanisms for the absorption of impacts or jolts, which can cause a change in position of the electrode and the danger of a breakthrough of the furnace. For example, if scrap is used, particularly individual pieces of scrap, the scrap can fall, hitting the end surface of the wear part of the bottom electrode and forcing it axially out of the bottom of the vessel.
Accordingly, it is an object of the present invention to avoid the above-mentioned disadvantages and create a metallurgical vessel which has a structurally simple, strong, wear-resistant bottom electrode which can be easily replaced and in which the essential parts can be reused.